1. Field of the Invention
This invention relates to an electrode material for semi-conductor devices, and particularly to an electrode material suitable for the production of semiconductor elements having an electrode in a relatively rough pattern, such as solar cells, etc.
2. Description of the Prior Art
Typical structure of a solar cell as a semi-conductor element is shown in the accompanying drawing, where light recipient side electrodes 4 and a back side electrode 5 are formed on the light recipient side and the back side, respectively, of a Si substrate 1 having an n.sup.+ layer 2 and a p.sup.+ layer in an n.sup.+ /p/p.sup.+ junction, and generally antireflective layers, etc. can be further provided on it.
Recently, an important task of the solar cell has been a reduction of production cost, and use of a low cost plating process or printing process has been investigated in place of the conventional vacuum vapor deposition process for forming the light recipient side electrode 4 and the back side electrode 5 in the FIGURE and above all the printing process has been extensively investigated owing to easy automation and high productivity. According to the printing process, a pasty mixture of metal powders, glass powders, etc. kneaded with an organic binder and an organic solvent (which will be hereinafter referred to as "electroconductive paste") is applied to a substrate by screen printing, etc., and fired, where generally silver powders are used as the metal powders. Many kinds of such electroconductive paste are commercially available for forming electrodes of solar cell or for forming a thick film circuit.
On the other hand, requirements for forming electrode of solar cell are high bonding strength of electrode, low contact resistance with silicon, no break-through of diffusion layer (small leak current), etc.
However, as a result of investigation of various, commercially available electroconductive pastes based on Ag or Ag-Pd, the present inventors have found the following problem when any of the electroconductive pastes is applied to silicon wafers with junctions as shown in FIGURE by printing, dried and fired. That is, a barrier is formed between the silicon wafer and the electrode in the case of an electroconductive paste based on Ag or Ag-Pd for forming a thick film circuit, resulting in an increase in contact resistance. Firing at a relatively high temperature damages the junction, resulting in an increase in leak current. Some of the electroconductive pastes based on Ag for forming a solar cell electrode hardly forms a barrier between the silicon wafer and the electrode, but the contact resistances are all high. Investigation of current-voltage characteristics of a solar cell at light irradiation shows a small fill factor, and no efficient solar cells have not been available.
High firing temperature tends to lower the contact resistance, but the leak current is increased. Thus, it has been very difficult to form electrodes with a low contact resistance from the conventional electroconductive pastes as mentioned above without any damage of junctions.